I don't know the biology of it, but I can vouch for the OP here. Being forced to spin at high speeds after a hard climb does flush the lactic acid (or whatever) out of you leg muscles very well. All of us who have done crazy descents after hard, hard fix gear climbs know this. Don't believe me? Try it. And if you live in the flats, you could put your bike on a trainer, spend 10 minutes going 25 on a 53-13 out of the saddle, then sit down, turn the motor you have hooked up to your crankset on and spin the cranks at 180+ RPM for 4 minutes. (Make sure your cleat/pedal connection is really secure!)

You are right. Coasting doesn't cause the soreness. But allowing the lactic acid (or whatever) to sit in idle muscles does. And after that little test, you will believe us! The place where this really plays out is riding in rolling country. Typically, I hit the bottom of the hills well behind the geared guys who passed me going much faster about half way down. They carry their speed say one third of the next hill, then start to bog down and shift down. I hit the bottom of the hill going slower but spinning like crazy but usually pass them about as they are shifting. Sometimes this goes on for miles, passing and getting passed by the same riders. At the end of the day, we feel quite different. They feel beat up by the hills. I feel drained but not sore. (OF course, there are the hills that are too hard. Last year's Cycle Oregon had a 2mile climb with two stretches of 14%. I did it on the 42-17. When I rolled into camp, I was sore! (Especially my arms. Showering, it hurt to touch them with the bar of soap! But thank G** for the downhills! At least my legs got spun out.)

Ben